1. Field of the Invention
The present invention generally relates to thin film photovoltaic devices. More particularly, it relates to copper indium gallium diselenide/disulfide—(CIGS)-based thin film photovoltaic devices.
2. Description of the Related Art including information disclosed under 37 CFR 1.97 and 1.98
In order to be commercially viable, photovoltaic (PV) cells must generate electricity at a competitive cost to fossil fuels. To meet these costs, the PV cells must comprise low cost materials along with an inexpensive device fabrication process and with moderate to high conversion efficiency of sunlight to electricity. In order for a device-building method to succeed, the materials synthesis and device fabrication must be commercially scalable.
At present, the photovoltaic market is still dominated by silicon wafer-based solar cells (first-generation solar cells). However the active layer in these solar cells comprises silicon wafers having a thickness ranging from microns to hundreds of microns because silicon is a relatively poor absorber of light. These single-crystal wafers are very expensive to produce because the process involves fabricating and slicing high-purity, single-crystal silicon ingots, and is also very wasteful.
The high cost of crystalline silicon wafers has led the industry to look at cheaper materials to make solar cells and, for this reason, much development work has focused on producing high efficiency thin film solar cells where material costs are significantly reduced compared to silicon.
Semiconductor materials like copper indium gallium diselenides and sulfides (Cu(In,Ga)(S,Se)2, herein referred to as “CIGS”) are strong light absorbers and have band gaps that match well with the optimal spectral range for PV applications. Furthermore, because these materials have strong absorption coefficients the active layer in the solar cell need be only a few microns thick.
Copper indium diselenide (CuInSe2) is one of the most promising candidates for thin film PV applications due to its unique structural and electrical properties. Its band gap of 1.0 eV is well-matched with the solar spectrum. CuInSe2 solar cells can be made by selenization of CuInS2 films because, during the selenization process, Se replaces S and the substitution creates volume expansion, which reduces void space and reproducibly leads to a high quality, dense CuInSe2 absorber layers. [Q. Guo, G. M. Ford, H. W. Hillhouse and R. Agrawal, Nano Lett., 2009, 9, 3060] Assuming complete replacement of S with Se, the resulting lattice volume expansion is approximately 14.6%, which is calculated based on the lattice parameters of chalcopyrite (tetragonal) CuInS2 (a=5.52 Å, c=11.12 Å) and CuInSe2 (a=5.78 Å, c=11.62 Å). This means that the CuInS2 nanocrystal film can be easily converted to a predominantly selenide material by annealing the film in a selenium-rich atmosphere. Therefore, CuInS2 is a promising alternative precursor for producing CuInSe2 or CuIn(S,Se)2 absorber layers.
The theoretical optimum band gap for absorber materials is in the region of 1.2-1.4 eV. By incorporating gallium into CuIn(S,Se)2 thin films, the band gap can be manipulated such that, following selenization, a CuxInyGazSaSeb absorber layer is formed with an optimal band gap for solar absorption.
Conventionally, costly vapor phase or evaporation techniques (for example metalorganic chemical vapor deposition (MO-CVD), radio frequency (RF) sputtering, and flash evaporation) have been used to deposit the CIGS films on a substrate. While these techniques deliver high quality films, they are difficult and expensive to scale to larger-area deposition and higher process throughput. Thus, solution processing of CIGS materials has been explored. One such approach involves depositing CIGS nanoparticles, which can be thermally processed to form a crystalline CIGS layer.
One of the major advantages of using CIGS nanoparticles is that they can be dispersed in a medium to form an ink that can be printed on a substrate in a similar way to inks in a newspaper-like process. The nanoparticle ink or paste can be deposited using low-cost printing techniques such as spin coating, slit coating and doctor blading. Printable solar cells could replace the standard conventional vacuum-deposited methods of solar cell manufacture because the printing processes, especially when implemented in a roll-to-roll processing framework, enables a much higher throughput.
The synthetic methods of the prior art offer limited control over the particle morphology, and particle solubility is usually poor which makes ink formulation difficult.
The challenge is to produce nanoparticles that overall are small, have low melting point, narrow size distribution and incorporate a volatile capping agent, so that they can be dispersed in a medium and the capping agent can be eliminated easily during the film baking process. Another challenge is to avoid the inclusion of impurities, either from synthetic precursors or organic ligands that could compromise the overall efficiency of the final device.
U.S. Pat. No. 8,784,701 [Preparation of Nanoparticle Material, published 4 Jun. 2009] and commonly-owned U.S. patent application Ser. No. 61/772,372 [Nanoparticle Precursor for Thin-Film Solar Cells, filed 4 Mar. 2013] describe the synthesis of colloidal CIGS nanoparticles having a monodisperse size distribution, capped with organic ligands that enable solution processibility and that can be removed at relatively low temperatures during thermal processing.
One of the challenges associated with the nanoparticle-based CIGS deposition approach is to achieve a high “crack-free limit” (CFL). The high organic content of colloidal CIGS nanoparticle-based ink formulations leads to large volume reduction when the as-deposited films are thermally processed. This reduction in volume can lead to cracking, peeling and delamination of the film. The critical thickness to which a film can be coated without this happening is known as the CFL. For colloidal CIGS nanoparticles, the CFL is typically about 100-150 nm, therefore ten or more coatings may be required to form a sufficiently thick film for a PV device.
Approaches to increase the CFL of colloidal nanoparticle films for optoelectronic device applications have been investigated. One such strategy is to reduce the organic content of the ink formulation, which can be achieved by synthesising nanoparticles with short-chain ligands or replacing the ligands with shorter chain functionalities, for example using a ligand exchange process. For example, Wills et al. reported the exchange of oleate ligands with shorter chain octyldithiocarbamate ligands on the surface of PbSe/CdSe core/shell nanoparticles to prepare more densely packed nanoparticle films. [A. W. Wills, M. S. Kang, A. Khare, W. L. Gladfelter and D. J. Norris, ACS Nano, 2010, 4, 4523] However, ligand exchange adds an extra processing step to the nanoparticle synthesis, and complete exchange can be difficult to achieve. Using the alternative approach of passivating the nanoparticle surface with short-chain ligands during colloidal synthesis requires changes to the reaction chemistry, and can lead to aggregation of the nanoparticles, rendering them poorly soluble.
In the ceramics industry, it is known that organic additives such as binders can be incorporated into a precursor solution to increase its CFL. However, this is unfavorable for CIGS nanoparticle films, since the organic additives may decompose to leave carbon residues within the film that can be detrimental to device performance. For example, Oda et al. reported a reduction in cracking of CuGaSe2 films produced via an electro-deposition process with the addition of gelatin to the precursor solution, however the post-annealing carbon concentration was found to increase with increasing gelatin concentration. [Y. Oda, T. Minemoto and H. Takakura, J. Electrochem. Soc., 2008, 155, H292] Further, in the preparation of solution processed CIGS films, additives such as binders typically decompose at particle surfaces, which can impede grain growth. [T. Todorov and D. B. Mitzi, Eur. J. Inorg. Chem., 2010, 1, 17] An additional method used in the ceramics industry is to increase the drying time to prevent rapid film shrinkage, however this also increases the processing time.
Thus, there is a need for a method that increases the CFL of CIGS nanoparticle films, without substantially increasing the processing time or introducing components into the film that would be detrimental to device performance and/or impede grain growth.